Investigation Of Murine Esophageal Myogenesis
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This work focuses on the development of the muscular layers of the murine esophagus. Considerable controversy surrounds the manner in which the muscularis externa of the esophagus develops, ever since investigators proposed the rare phenomenon of transdifferentiation as a mechanism for virtually complete replacement of smooth muscle by striated muscle that is observed in the murine esophagus. I found, using several independent levels of evidence, that transdifferentiation is an unlikely explanation for esophageal myogenesis. First, by fate-mapping smooth muscle cells, I failed to observe expression of the fate-mapping marker in any esophageal striated muscle. Second, I failed to co-localize expression of smooth-muscle-specific and striated-muscle-specific proteins in any cell undergoing differentiation into a striated muscle lineage. Finally, selective deletion of a critical striated muscle transcription factor (myogenin) in esophageal smooth muscle cells prior to the appearance of any striated muscle failed to arrest subsequent esophageal striated myogenesis. To address prior concerns of the fate of esophageal smooth muscle, I examined smooth muscle morphology in transgenic mice expressing smoothmuscle-specific eGFP, and identified 3 possible fates: (A) the incorporation of smooth muscle cells into the striated muscle stroma as ?remnant? smooth muscles; (B) compaction and persistence of smooth muscle cells in the pre- gastric region; and (C) occasional apoptosis of smooth muscle cells. Using exhaustive co-immunolabeling methods, I demonstrated that remnant smooth muscle cells are indeed a previously unidentified cell population resident within the striated esophageal musculature. Finally, I examined the origin of striated muscle precursors in a series of ex vivo experiments. I was able to maintain a functional esophageal phenotype in culture for up to 7 days, and determined that striated muscle precursors most likely populate the esophageal muscularis externa early in development, and are subsequently induced to express a myogenic program in a craniocaudal direction. Together, these findings extend the understanding of esophageal myogenesis and redress fundamental concepts of myogenesis, namely, transdifferentiation, by providing evidence of a more simple, classical myogenic program in this unique muscular organ.
dissertation or thesis